Japan’s biggest opposition party failed to win any of the five seats up for grabs in Tokyo after a dismal campaign that seemed doomed from the start. The party had two of the seats before Sunday’s poll, but decided to field only one candidate, 49-year-old Kan Suzuki, an incumbent, effectively losing one seat before proceedings had even begun.

Adding to the DPJ’s pain, voters opted for political greenhorns over Mr. Suzuki, including Taro Yamamoto, an independent candidate who has more experience making movies than drafting legislature.

“I have been getting a good response” as the campaign progressed, said Mr. Yamamoto, a former actor-turned-nuclear activist, after the initial exit polls released at 8 p.m. indicated he was headed for victory. About an hour later, state-broadcaster NHK declared the 38-year-old anti-nuclear advocate the winner.

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Democratic Party of Japan lawmaker Kan Suzuki, right, shown during happier times back when he won a Tokyo seat in the 2009 upper House election.

It’s another harsh rebuke for the DPJ, following its disastrous loss in the December general election, when voters unceremoniously tossed it back into opposition after the party failed to live up to its campaign pledges during its three-year stint in power.

“The DPJ had a lot of momentum behind it in the last (2009) election, but once in power, they disappointed us. They promised us so many nice things, but they couldn’t deliver,” said Seigo Asano, a small business owner in Tokyo on Sunday. Mr. Asano said he had voted for the DPJ in the last upper house election in 2010, but cast his ballot for the ruling Liberal Democratic Party this time around. “I voted for the LDP because it’s the party that is doing most for the economy.”

Incumbent candidates Tamayo Marukawa of the LDP and Natsuo Yamaguchi, the head of the LDP coalition partner, New Komeito, were declared early victors according to exit poll projections. Mr. Yamamoto was deemed the third winner about an hour later.

This election has been much kinder to Mr. Yamamoto. His first attempted foray into politics ended in failure when he lost his race in the lower house election in December despite a spirited campaign. His change in fortune may in part be due to a change in election laws that no longer bar candidates from using the Internet during the official campaign period.

“It has been a tailwind for us,” said Mr. Yamamoto, a voracious user of social media who often live streams his campaign events.

Mr. Yamamoto became an outspoken advocate to end Japan’s reliance on nuclear power following the 2011 Fukushima Daiichi nuclear accident, a passion that sparked his interest to enter politics.

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